gaearon

Note:

When this guide is more complete, the plan is to move it into Prepack documentation.
For now I put it out as a gist to gather initial feedback.

A Gentle Introduction to Prepack (Part 1)

If you're building JavaScript apps, you might already be familiar with some tools that compile JavaScript code to equivalent JavaScript code:

• Babel lets you use newer JavaScript language features, and outputs equivalent code that targets older JavaScript engines.

developit

const MODULE_DIR = /(.*([\/\\]node_modules|\.\.)[\/\\](@[^\/\\]+[\/\\])?[^\/\\]+)([\/\\].*)?$/g;

{
	loader: 'babel-loader',
	test: /\.jsx?$/,
	include(filepath) {
		if (filepath.split(/[/\\]/).indexOf('node_modules')===-1) return true;
		let pkg, manifest = path.resolve(filepath.replace(MODULE_DIR, '$1'), 'package.json');
		try { pkg = JSON.parse(fs.readFileSync(manifest)); } catch (e) {}
		return !!(pkg.module || pkg['jsnext:main']);

markerikson

[02:06 PM] acemarke: @Steven : a couple other thoughts on the whole NODE_ENV thing. First, per my comments, it really is a Node concept. It's a system environment variable that Node exposes to your application, and apparently the Express web server library popularized using its value to determine whether to do optimizations or not
[02:08 PM] acemarke: Second, because of its use within the Node ecosystem, web-focused libraries also started using it to determine whether to they were being run in a "development" environment vs a "production" environment, with corresponding optimizations. For example, React uses that as the equivalent of a C #ifdef to act as conditional checking for debug logging and perf tracking. If process.env.NODE_ENV is set to "production", all those if clauses will evaluate to false.
Third, in conjunction with a tool like UglifyJS that does minification and removal of dead code blocks, a clause that is surrounded with if(process.env.NODE_ENV !== "development")

paulirish

What forces layout / reflow

All of the below properties or methods, when requested/called in JavaScript, will trigger the browser to synchronously calculate the style and layout*. This is also called reflow or layout thrashing, and is common performance bottleneck.

Generally, all APIs that synchronously provide layout metrics will trigger forced reflow / layout. Read on for additional cases and details.

Element APIs

Getting box metrics

• elem.offsetLeft, elem.offsetTop, elem.offsetWidth, elem.offsetHeight, elem.offsetParent

paulirish

// find out what prefix this browser supports.

// usage: gimmePrefix('transform') // 'WebkitTransform'
//        returns false if unsupported.

function gimmePrefix(prop){
  var prefixes = ['Moz','Khtml','Webkit','O','ms'],
      elem     = document.createElement('div'),
      upper    = prop.charAt(0).toUpperCase() + prop.slice(1);